Impedance Conversion for Musical Instruments

Active Plug Schematic Diagram

I have developed an FET impedance converter for use with the Fishman pickup on my Kay orchestra bass. As shown in the schematic below, the circuit is divided into two sections, so that the input section can be mounted inside the head shell of a 1/4" plug. This puts the active circuitry right were it counts, on the instrument. Two isolated output circuits are shown, but you can have more. The prototype I use in performance has three outputs: 1/4" phone, RCA phono, and XLR. For the XLR output, I used a TRW/UTC #O-2 mic-line transformer, inside a #O-17 magnetic shield, to get even lower output impedance. For an improved version of this circuit with better headroom and reduced harmonic distortion, see my Active Plug Bootstrap page. For applications where battery power isn't required, I've also developed a Vacuum Tube Direct Interface, using triaxial cable to connect to the instrument.

Active Plug Schematic Diagram

The circuit is optimized for low current drain with a single 9 Volt power supply. Distortion is 1% or less up to 1 Volt RMS output. Gain is set at unity, which makes the circuit very stable with transient inputs, and matches up well with most instrument and sound reinforcement amplifier inputs. With no equalization provided, frequency response is flat allowing the circuit to be used with almost any instrument. Equalization, if needed, can be done on the mixing board or instrument amplifier.

Construction Details

Here are some snapshots of the JFET input circuit mounted on a Switchcraft SilentPlug 1/4" phone plug, and of the battery box with additional circuitry and mic-line transformer.

SilentPlug Details

Battery Box Details

Background Information

Musical instruments are usually equipped with high impedance transducers in one of several broad categories:

Resources on the Web

Here are some web sites with good information for musicians:


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